1. Cross-Reference to Related Applications
Reference is made to U.S. patent application Ser. No. 724,059, entitled "Method and Apparatus for Scanning a Receiving Medium," filed on Jul. 1, 1991, in the names of S. H. Baek et al.; Ser. No. 724,061, entitled "Method and Apparatus for Scanning a Receiving Medium," filed on Jul. 1, 1991, in the names of Baek, Mackin, Firth, and Woo; and Ser. No. 724,060, entitled "Method and Apparatus for Scanning a Receiving Medium," filed on Jul. 1, 1991, in the names of Mackin et al. Reference is also made to U.S. patent application Ser. No. 749,031, entitled "A Method of Calibrating a Multichannel Printer," filed on even date herewith in the name of Sanger et al. All of these applications are assigned to the assignee of the present invention.
2. Field of the Invention
The present invention relates to the calibration of a multichannel printer, and more particularly, to the calibration of such a printer in order to eliminate artifacts in an image produced thereby.
3. State of the Prior Art
In one type of scanner apparatus, a photosensitive material is supported on a rotatable drum, and a print head carrying a light source is advanced relative to the photosensitive material by means of a lead screw. The light source is modulated in accordance with an information signal to form an image on the photosensitive material. In order to increase the output of such apparatus, multiple light sources are mounted in the print head so that a plurality of print lines can be formed in a single pass. In multiline scanning systems, any difference of densities among the lines can create very severe artifacts in the image. These artifacts can appear as repetitive patterns known as "banding."
The problem of banding can be particularly troublesome in half-tone printing where, for example, 12 mini-pixels are used to write a half-tone dot. Visible lines in the image, caused by unevenness in the densities of the lines, can come at a different section of each successive half-tone dot, and thus, cycle across the image. The visible lines can be due, for example, to a variation in the intensity of the light sources. The frequency of the visible lines in the image beats with the half-tone dot frequency. The resulting macro density variation can have a spatial frequency in the image which, unfortunately, matches the frequency at which the eye is most sensitive, that is, at about 0.5 cycle/mm. At this frequency range, the typical human eye can see a variation of density of around 0.2%. This small level of unevenness in density is very hard to control in a printer using a multiline print head.
The aforementioned U.S. Patent Applications are directed to solutions to the problem of artifacts in images created by printers. In certain of the applications, random noise is introduced into the control signals of the printer in order to mask artifacts in an image produced by the printer. Other techniques have been used in the art to overcome the problems of image artifacts. For example, certain multiline film writers are capable of printing half-tone images, without artifacts, by using a high gamma film under saturated exposure conditions. Another technique which is used to mask artifacts is to vary the resolution of the film writer. Changing the line-to-line pitch and the line width of the film writer permit a selection of resolution which will limit artifacts at the desired half-tone ruling and screen angle. A disadvantage of increasing resolution is that it also increases both the amount of data required and the time to make each image.
Multiple writing beams can be formed from a single light single source by using beam splitter optics. Such a system will typically write six lines at a time. In these systems, the problem of balancing each writing line may be less severe, since the same light source is used for all of the lines; however, the intensity of light provided to each line is necessarily considerably reduced. None of the known prior-art solutions to the problem of artifacts in images produced by multiline printers is totally satisfactory, however, particularly when the writing is being done on mediums such as a thermal print medium.